Crystallization studies of poly(trimethylene terephthalate) using thermal analysis and far‐infrared spectroscopy

Crystallization of poly(trimethylene terephthalate) (PTT) by annealing was examined using density measurement, differential scanning calorimetry, and far‐infrared spectroscopy (FIR). Crystallinity, measured by density, increased slowly up to the T_a of 185 °C and increases rapidly once T_a exceeds 185 °C. It was found that thermally induced crystallization is mainly temperature‐dependent above T_a = 185 °C and temperature‐ and time‐dependent below T_a = 60 °C. Two melting transitions, T and T, were observed for those samples annealed above 120 °C. No significant change in T was observed as a function of T_a while T showed strong dependency on T_a. Digital subtraction of the amorphous contribution from the semicrystalline FIR spectra provided characteristic spectra of amorphous and crystalline PTT. The bands at 373, 282, and 92 cm^?1 were assigned to the crystalline phase, while the bands at 525, 406, and 351 cm^?1 were attributed to the amorphous phase. It was shown that FIR spectroscopy can be used as a means to estimate the degree of crystallinity of PTT. The band ratio of 373 and 501 cm^?1 was plotted against crystallinity measured by density and reasonably good correlation was obtained. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1675–1682, 2007     

Oxygen‐barrier properties of cold‐crystallized and melt‐crystallized poly(ethylene terephthalate‐co‐4,4′‐bibenzoate)

This study examined the oxygen‐transport properties of poly(ethylene terephthalate‐co‐bibenzoate) (PETBB55) crystallized from the melt (melt crystallization) or quenched to glass and subsequently isothermally crystallized by heating above the glass‐transition temperature (cold crystallization). The gauche–trans conformation of the glycol linkage was determined by infrared analysis, and the crystalline morphology was examined by atomic force microscopy. Oxygen solubility decreased linearly with volume fraction crystallinity. For melt‐crystallized PETBB55, extrapolation to zero solubility corresponded to an impermeable crystal with 100% trans glycol conformations, a density of 1.396 g cm^?3, and a heat of melting of 83 J g^?1. From the melt, PETBB55 crystallized as space‐filling spherulites with loosely organized lamellae and pronounced secondary crystallization. The morphological observations provided a structural model for permeability consisting of impermeable platelets randomly dispersed in a permeable matrix. In contrast, cold‐crystallized PETBB55 retained the granular texture of the quenched polymer despite the high level of crystallinity, as measured by the density and heat of melting. Oxygen solubility decreased linearly with volume fraction crystallinity, but zero solubility corresponded to an impermeable defective crystal with a trans fraction of 0.83 and a density of 1.381 g cm^?3. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2489–2503, 2002     

Structural studies on linear aliphatic polyesters by electron diffraction,infrared, and Raman spectroscopy

Infrared and Raman spectra of linear aliphatic polyesters synthesized by polycondensation of α,ω-diols and α,ω-dicarboxylic acids have been measured in the crystalline and amorphous states to study the crystallinity and conformation of the polyester chains.The repeating unit of crystalline polyesters with an even number of methylene groups in the alcoholic or acidic part and with a planar conformation of the chain exhibits a centre of inversion. In this special case the spectroscopic rule of mutual exclusion is followed: vibrations that are infrared-active are Raman-inactive, and vice versa.Comparison of the infrared and Raman spectra of a large number of polyesters showed that this rule is applicable in all cases but one. Presently, the only exception is polyester-2, 4 (ethanediol-succinic acid) where for almost all infrared bands a Raman line can be found. Consequently we have to assume that the chain of this polymer in some way deviates from planarity. Since typical infrared progression bands appeared in the region of the methylene wagging (1400-1200cm^?1) and rocking vibrations (910-720 cm^?1) the methylene sequences must have an essentially planar zigzag conformation. Thus, non-planarity should be due to the plane of the ester groups between methylene sequences having an angle other than zero with respect to the planes of adjacent methylene sequences. Some of the polyesters were also studied by electron diffraction. The results of these examinations support the assumption of a planar conformation of the chains of almost all the polyesters studied.     

Crystalline transition of polyamide‐10,20 investigated by in situ Fourier transform infrared spectroscopy

Real‐time Fourier transform infrared spectroscopy was used to investigate the crystalline transition of even–even polyamide‐10,20. During the crystalline transition, the intensity of the hydrogen bonds became weak, the twisting of the C? CO and C? N bonds improved, and the gauche conformation of the CH₂ sequences increased, along with the strengthening of the vibration of the methylene units. The ordered stacking of methylene segments with the trans‐zigzag conformation gradually became disordered by the insertion of the gauche conformation upon heating. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4017–4022, 2004     

Conformation of drawn poly(trimethylene terephthalate) studied by solid-state 13C NMR

The change in the conformation of the flexible O-CH2-CH2-CH2-O segment of poly(trimethylene terephthalate) (PTT) monofilament caused by drawing was investigated by means of the gamma-gauche effect on the 13C solid-state NMR chemical shift of the internal methylene carbon, combined with the NMR relaxations. The conformation around the O-CH2 and CH2-O bonds for as-spun fiber was trans/trans. On drawing, followed by heat treatment, the conformation changed to gauche/gauche. The ratio of gauche/gauche to trans/trans for the drawn PTT fiber was determined quantitatively.     

Phase structure of electrospun poly(trimethylene terephthalate) composite nanofibers containing carbon nanotubes

Nanofibrous composite mats were prepared by electrospinning of poly(trimethylene terephthalate), PTT, with multi-walled carbon nanotubes (PTT/MWCNT). Trifluoroacetic acid (TFA) and methylene chloride (MC) with volume ratio of 50/50 is a good solvent for PTT and was used as the electrospining solution. Scanning electron microscopy was used to investigate the morphology of electrospun (ES) nanofibers with 0, 0.2, 1.0, or 2.0 wt% of MWCNTs. Crystal structure of the ES mats was determined from wide angle X-ray diffraction. Thermal properties were investigated using heat capacity measurements from differential scanning calorimetry (DSC) using the three-runs method for baseline correction, heat flow amplitude calibration, and sample heat capacity determination. A model comprising three phases, a mobile amorphous fraction (MAF), rigid amorphous fraction (RAF), and crystalline fraction (C), is appropriate for ES PTT/MWCNT fibers. The phase fractions, W _i (for i = RAF, MAF or C) were determined by DSC. Crystallinity decreases very slightly with the amount of MWCNT. At the same time, a large increase in RAF was observed: W _RAF of PTT fiber with 2% MWCNT is twice that of neat PTT fiber. The addition of MWCNTs enhanced the PTT chain alignment and increased RAF as a result. Changes of vibrational band absorbance at 1358 and 1385 cm⁻¹, corresponding to characteristic groups, were obtained with infrared spectroscopy. The increased absorbance at 1358 cm⁻¹ and decreased absorbance at 1385 cm⁻¹, with the addition of MWCNTs, strongly supports the three-phase model for ES PTT/MWCNT nanocomposites.     

Spectroscopic studies of 1,4-dibromobutyne-2

Infrared spectra of 1,4-dibromobutyne-2 have been recorded over the 4000-200 cm^?1 region in the vapour, liquid, amorphous and crystalline states Raman spectra were extended to ca. 20 cm^?1 in the same states of aggregation, except for the non-recorded vapour phase spectrum. The temperature was varied between ?190 and 160 °C, and the pressure up to 10 kbar.A high proportion of the molecules exhibited free, internal rotation in the vapour and liquid phases, but to a smaller extent in the amorphous state at ?190 °C. For those molecules not being excited beyond the potential barrier, an unsymmetric conformation was preferred, whereas in the crystalline state the molecules possessed the anti conformation (C_2h) both at low temperature and at high pressure at ambient temperature.A vibrational analysis based upon force field calculations was carried out and the mean amplitudes of vibration computed. The data have been related to preliminary results from dipole moment and electron diffraction investigations.     

聚对苯二甲酸乙二酯非晶区反式构象结构的探讨

<正> 聚对苯二甲酸乙二酯(PET)的重复单元中有乙撑基。它的碳碳键内旋转产生反式构象(T)和左右式构象(G)。用其不同的构象来表征PET的聚集态结构,对研究结构与性能的关系是十分重要的。许多工作都已指出,乙撑基反式构象的谱带热处理时随结晶度的增加而增加。X-射线衍射的结果证实     

用傅里叶变换红外光谱法研究聚对苯二甲酸乙二酯的结构

用傅里叶变换红外光谱法,将一系列不同温度下退火的PET光谱与熔融淬火的PET光谱进行数字吸光度相减,得到不同结晶状态的“反式构象光谱”。进一步将不同温度退火的“反式构象光谱”(240℃和120℃)彼此相减,可得到“纯晶区的光谱”和“非晶区的反式构象光谱”。通过差示光谱的比较,发现一些新的谱带和谱带的变化。可用PET 的晶区和非晶区中其分子链的内旋转构象的变化解释。     

Infrared study of high-pressure crystallization of polyethylene

The high-pressure crystallization of polyethylene in a diamond cell has been studied by infrared spectroscopy. The splitting of the CH₂ rocking band at 720–730 cm^?1 as a function of pressure was analyzed. It was found that pressure alone up to 3 kbar will not change the distance between methylene groups in the unit cell. However, this distance can be shortened by crystallization at this pressure. Intensities of selected crystalline (1176 and 1050 cm^?1) and amorphous (1303, 1352, and 1368 cm^?1) bands were measured on samples before and after high-pressure crystallization, and also on samples of various densities crystallized under atmospheric pressure. The increase in the intensities of crystalline bands and concomitant decrease in amorphous bands, together with density changes, indicate that the crystallinity can be enhanced by crystallization under high pressure. Nevertheless, the crystallinity of polyethylene crystallized at high pressure is comparable with that of polyethylene crystallized at atmospheric pressure at low undercooling for long periods of time.     

Structural relaxation in amorphous 1,2-dichloroethane studied by transformation between conformation isomers

Structural relaxation in amorphous 1,2-dichloroethane (DCE) samples prepared by vapor deposition on cold substrates were studied by Raman scattering. The gauche and trans molecules of DCE were found to coexist in amorphous states immediately after the deposition, and structural relaxation occurred with temperature elevation before crystallization. Mole fraction of the gauche isomer increased during this relaxation process, although trans is the stable isomer in gaseous and crystalline states. At the final amorphous stage immediately before crystallization, the gauche mole fraction was close to the mole fraction of the supercooled liquid state. It was also found that trans molecules located at positions with lower density were more easily transformed into the gauche conformation, while the distribution of the local structure around the resultant gauche molecules remained almost unchanged during the structural relaxation. Such behaviors of amorphous DCE are discussed from the viewpoint of the characteristic molecular structure of DCE.     

Heat capacity of poly(trimethylene terephthalate)

The heat capacity of poly(trimethylene terephthalate) (PTT) has been measured using adiabatic calorimetry, standard differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC). The heat capacities of the solid and liquid states of semicrystalline PTT are reported from 5 to 570 K. The semicrystalline PTT has a glass transition temperature of 331 K. Between 340 and 480 K, PTT can show exothermic ordering depending on the prior degree of crystallization. The melting endotherm of semicrystalline samples occurs between 480 and 505 K, with a typical onset temperature of 489 K (216°C). The heat of fusion of the semicrystalline samples is about 15 kJ mol⁻¹. For 100% crystalline PTT the heat of fusion is estimated to be 30 ± 2 kJ mol⁻¹. The heat capacity of solid PTT is linked to an approximate group vibrational spectrum and the Tarasov equation is used to estimate the heat capacity contribution due to skeletal vibrations (θ₁ = 550.5 K and θ₂ = θ₃ = 51 K, N_skeletal = 19). The calculated and experimental heat capacities agree to better than ±3% between 5 and 300 K. The experimental heat capacities of liquid PTT can be expressed by: $ C^L_p(exp) $ = 211.6 + 0.434 T J K⁻¹ mol⁻¹ and compare to ±0.5% with estimates from the ATHAS data bank using contributions of other polymers with the same constituent groups. The glass transition temperature of the completely amorphous polymer is estimated to be 310–315 K with a ΔC_p of about 94 J K⁻¹ mol⁻¹. Knowing C_p of the solid, liquid, and the transition parameters, the thermodynamic functions enthalpy, entropy, and Gibbs function were obtained. With these data one can compute for semicrystalline samples crystallinity changes with temperature, mobile amorphous fractions, and resolve the question of rigid-amorphous fractions.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2499–2511, 1998     

Two‐dimensional Fourier transform infrared spectroscopy study of biosynthesized poly(hydroxybutyrate‐co‐hydroxyhexanoate) and poly(hydroxybutyrate‐ co‐hydroxyvalerate)

Generalized two‐dimensional (2D) Fourier transform infrared correlation spectroscopy was used to investigate the effect of the comonomer compositions on the crystallization behavior of two types of biosynthesized random copolymers, poly(hydroxybutyrate‐co‐hydroxyhexanoate) and poly(hydroxybutyrate‐co‐hydroxyvalerate). The carbonyl absorption band around 1730 cm^?1 was sensitive to the degree of crystallinity. 2D correlation analysis demonstrated that the 3‐hydroxyhexanoate units preferred to remain in the amorphous phase of the semicrystalline poly(hydroxybutyrate‐co‐hydroxyhexanoate) copolymer, resulting in decreases in the degree of crystallinity and the rate of the crystallization process. The poly(hydroxybutyrate‐co‐hydroxyvalerate) copolymer maintained a high degree of crystallinity when the 3‐hydroxyvalerate fraction was increased from 0 to 25 mol % because of isodimorphism. The crystalline and amorphous absorption bands for the carbonyl bond for this copolymer, therefore, changed simultaneously. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 649–656, 2002; DOI 10.1002/polb.10126     

Poly(ethylene terephthalate) copolymers containing nitroterephthalic units. II. Crystallization and conformational studies

The effect of incorporating a nitro side group into the phenylene units of poly(ethylene terephthalate) (PET) on the conformation and crystallizability of this polyester was evaluated. Random poly(ethylene terephthalate‐co‐nitroterephthalate) (PETNT) copolymers containing 5, 10, and 15 mol % nitroterephthalic units were investigated with reference to PET. All the examined copolymers were semicrystalline and were found to adopt the triclinic crystal structure of PET, with the nitrated units being excluded from the crystallites. Both the crystallinity and crystallization rate of PETNT largely decreased with the content of nitrated units, and the O? CH₂? CH₂? O trans‐to‐gauche conformational ratio increased with crystallization, attaining comparable values for all the compositions. The conformation and crystallinity of isothermally crystallized PET and PETNT samples could be correlated with the size of the crystallites generated in each case. However, a different crystal perfecting mechanism seemed to operate for PET and for the PETNT copolymers when they were subjected to annealing. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2759–2771, 2002     

用FTIR定量测定聚对苯二甲酸乙二酯晶区和非晶区反式构象含量

用傅里叶变换红外分光光度计测定不同条件下退火处理的聚对苯二甲酸乙二酯膜的谱图, 用谱带分离技术对归属于反式构象的1340cm^-^1谱带进行阻尼最小两乘分峰处理, 结合属左右式构象贡献的1454cm^-^1谱带, 用线性回归分析的方法获得晶区反式构象、非晶区反式构象、非晶区左右式构象的百分含量。     

Oxygen‐barrier properties of oriented and heat‐set poly(ethylene terephthalate)

The improvement in the oxygen‐barrier properties of poly(ethylene terephthalate) (PET) by orientation and heat setting was examined. Orientation was carried out at 65 °C by constrained uniaxial stretching to a draw ratio of about 4. Heat setting was performed at temperatures from 90 to 160 °C with the specimen taut. Orientation decreased the permeability of PET to almost one‐third that of the unoriented, amorphous polymer because of decreases in both the diffusion coefficient and the solubility coefficient. The proposed two‐phase model for oriented PET consisted of a permeable isotropic amorphous phase (density = 1.335 g/cm³) with ethylene linkages predominately in the gauche conformation and an impermeable oriented phase (density = 1.38 g/cm³) with ethylene linkages that had transformed from the gauche conformation to the trans conformation during stretching. Chain segments in the trans conformation did not possess crystalline order; instead, they were viewed as forming an ordered amorphous phase. Crystallization by heat setting above the glass‐transition temperature did not dramatically affect the permeability. However, a decrease in the diffusion coefficient, offset by an increase in the solubility coefficient, indicated that crystallization affected the barrier properties of the permeable amorphous phase. Analysis of the barrier data, assuming a two‐phase model with variable density for both the permeable and impermeable phases, revealed that the impermeable phase density increased during crystallization, approaching a value of 1.476 g/cm³. This value is consistent with previous measurements of the density of the defective crystalline phase in PET. The density of the permeable amorphous phase decreased concurrently to about 1.325 g/cm³, indicating the appearance of additional free volume. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1679–1686, 2000     

The chain conformations of a polyether based on bis(4-hydroxyphenoxy)-p-xylylene and 1,11-dibromoundecane (HPX-c11) in its mesophase states,derived from 13C NMR

A thermotropic main-chain polyether based on bis(4-hydroxyphenoxy)-p-xylylene and 1,11-dibromoundecane has been studied by variable-temperature solid-state ¹³C NMR. Between isotropization and glass transition temperatures, the material can be identified to be semicrystalline, consisting of two conformationally and motionally distinguishable phases. The more mobile component is liquid-like and thus, can be attributed to an amorphous phase. In the more rigid component, the molecules have a conformationally disordered methylene sequence. In the low-temperature ordered phase approximately ? of the carbon-carbon bonds are trans (t). Starting from the bond between the oxygen and the first methylene carbon, the bond conformations are: d? t? d? t? t? t? t? t? t? d? t? d, where d stands for disordered (i.e., it represents the common dynamic interchange between gauche and trans with an overall gauche content of perhaps 40%). The motion of the αα′-diphenoxy-p-xylylene unit consists mainly of 180° ring-flips, which cause no entropy increase relative to ordered phenylene groups in a crystal, but significantly changes the ¹³C NMR spectra. The central p-xylylene ring starts its flipping motion at a lower temperature than the two phenoxy rings. The high-temperature mesophase contains a methylene sequence of the bond conformations: d? t? d? d? d? t? t? d? d? d? t? d. Thus, the difference between the low-temperature and high-temperature mesophases consists of different degrees of conformational disorder. Thermal analysis seems to indicate that additional mesophases may be possible. © 1994 John Wiley & Sons, Inc.     

Application of fourier transform infrared spectroscopy to the study of semicrystalline polymers: Poly(ethylene terephthalate)

An infrared absorbance subtraction technique has been used to “isolate” bands in the composite spectrum of semicrystalline polymers according to their crystalline or amorphous character. Amorphous and crystalline spectra for annealed, melt-quenched, and solution-cast poly(ethylene terephthalate) have been separated. The spectra of the amorphous component show an increased intensity of bands associated with the trans configuration of oxygen about the C? C bond when the polymer is annealed. This increased “trans” band intensity reflects the increased proportion of trans structures as a result of annealing. The amorphous trans bands are shifted approximately 1–3 cm^?1 from their positions in the crystalline “trans” spectrum. The frequency shift of these bands can be attributed to the differences in chain interactions that exist in the amorphous phase and the crystalline lattice. We have also found that under identical anealing conditions the amorphous phase of the melt-quenched polymer contains an increased intensity of conformational trans bands compared to the sample cast from solution.     

Fourier-transform infrared study of coextrusion-drawn poly(ethylene terephthalate)

The effects of initial morphology and extrusion temperature on the orientational anisotropy and conformational changes on coextrusion drawing of poly(ethylene terephthalate) (PET) have been determined by Fourier-transform polarized infrared spectroscopy. The samples were drawn from both amorphous and semicrystalline (50%) PET at 50 and 90°C. A strong influence of coextrusion drawing temperature was observed for overall chain orientation evaluated from the dichroic ratio of the 795-cm^?1 band for the samples prepared from the amorphous state: this dependence was less prominent in samples drawn from the semicrystalline state. Under the same drawing conditions, the dichroic ratio for the 973-cm^?1 trans band for samples prepared from the amorphous state was higher than from the semicrystalline state. Furthermore, in all samples, the relative intensity of this band was almost proportional to the degree of crystallinity. In all samples, the gauche content, evaluated from the 896-cm^?1 band, decreased with increasing draw ratio. However, the dichroic ratio of this band was near unity regardless of draw ratio, initial morphology, or extrusion temperature. From these results it is considered that all gauche units in the amorphous phase are almost isotropic in the extrusion-drawn samples with overall orientation arising largely from the crystalline chains possessing totally the trans conformation (973 cm^?1) in its content. In order to evaluate the deformation mechanism of the coextrusion drawing method, the relationship between the bulk and film surface orientation is also reported.     

New directions of matrix ir-spectroscopy

By use of thermal molecular beam IR matrix spectroscopy and IR irradiation in selected frequency ranges the vibrational spectra of conformers of 1,2-difluoroethane and methylvinylether will be discussed. The former molecule affords a case, where by comparison of CO₂ laser and 3000 cm^?1 irradiation the barriers to interconversion gauche?trans may be estimated. Methylvinylether on the other hand is a molecule, whose conformers may be interconverted by a 1-photon process in the 3000 cm^?1 region, but not by photons with frequency lower than 1400 cm^?1.